Thus, one particular application field of the invention is resource sharing in frequency bands currently used in AM (Amplitude Modulation) emissions, in other words at long waves from 153 to 279 kHz, medium waves from 520 to 2500 kHz and short waves from 3.1 to 26.1 MHz. It is intended to gradually replace analogue services that are becoming less popular by much better quality digital services in order to revitalise listening to the radio in these frequency bands.
Thus, a digital radio broadcasting standard was defined and validated at the ETSI in September 2001, within the context of the DRM (“Digital Radio Mondiale”) project in which the holders of this patent application are participating. In this system, it is planned to transmit a radiophone service using compression, encoding and digital modulation techniques so that the signal characteristics can be better adapted to channel constraints in these frequency bands.
The transmission technique used is the COFDM already used in DAB (Digital Audio Broadcasting) and DVB-T (Digital Video Broadcasting-Terrestrial) standards.
In a transient phase for introduction of the DRM system, consortium partners and particularly broadcasting organisations would like to assure continuity of the existing AM service and implement simultaneous emissions combining a digital signal and a classical analogue AM signal that broadcast identical programs (“simulcast” technique).
In particular, it is planned to associated these two emissions by placing them in distinct channels at adjacent frequencies in the radio frequency spectrum.
As planned by the DRM standardisation consortium, the principle of the “simulcast” mode combines an AM analogue emission and a DRM digital emission placed in contiguous channels in the frequency spectrum according to different possible configurations as shown in FIG. 1.
In this Figure, the AM signal is represented by a triangle and the DRM signal is represented by a rectangle. The digital and analogue channels can occupy a variable passband that is a multiple of 4.5 kHz for medium waves and 5 kHz for short waves. They are adjacent in all cases.
This approach is suitable for the digital signal, for which reception can be assured efficiently. On the other hand, it raises problems with classical AM receivers, and particularly low range AM receivers for use by the general public that account for a large proportion of all receivers.
These receivers usually have poor selectivity and amplitude-frequency response performances.
As can be seen in FIG. 2, that shows selectivity groups for three different medium waves (MW) AM receivers with a spacing of less than 9 kHz between channels. It can be seen that these responses are not linear and go well beyond the limits of the AM channel, which is +/−4.5 kHz with respect to the central frequency Fc of the channel in medium wave.
As can be seen in FIGS. 3a and 3b, this passband 31 which is too wide in radio frequency, encompasses not only the AM signal 32, but also a variable sized portion of the adjacent digital channel 33.
As illustrated in FIG. 3b, the demodulated audio signal 34 contains a portion 35 of the digital signal in frequencies beyond 4.5 kHz. It is known that the ear is very sensitive at these frequencies. Thus, there is interference when listening to the audio signal.
This result is contrary to the objective fixed by the “simulcast” approach, which is to maintain the possibility for persons with a classical AM receiver to receive programs. Obviously, listeners will stop listening to these programs if they are affected by interference.
Obviously, it would be possible to attenuate interference by reducing the power level of the digital signal compared with the analogue AM signal. However in this case, the reduction in the level of the digital signal will cause a serious loss over the coverage area of the digital service.
Measurements of the protection ratio have shown that the digital signal has to be at least 16 dBc weaker than the analogue signal so as to guarantee a relatively acceptable interference level with a good proportion of low performance AM receivers. But even in this case, there are still situations in which AM reception remains severely disturbed.
Other approaches could be envisaged to simultaneously transmit the digital signal and the analogue signal. However, in all cases the digital signal will disturb the analogue signal, creating interference in low performance AM receivers.